This invention relates to electric motor and in particular, to a double insulated rotor for an electric motor such as a universal motor or high voltage d.c. motor.
Generally, wound rotors comprise a shaft and a rotor core fixed to the shaft. Rotor windings are wound around poles of the rotor core to form the armature. The power of the motor comes from the interaction between the magnetic fields of the stator and rotor and the rotational force applied to the rotor core is transferred to the shaft for doing useful work such as driving a load.
The rotor core may be fixed directly onto the shaft as a press fit which usually gives a strong connection for transferring the power or torque. However, in some applications, safety specifications require that the rotor core be electrically insulated from the shaft, known as a double insulated rotor. This is a common requirement for mains voltage motors.
One way this is achieved is by insert moulding an insulator between the rotor core and the shaft. In this process, the rotor core and shaft are loosely assembled and placed in an injection mould. The mould is closed and hot plastics resin is injected into the spaces between the rotor core and the shaft to fix the core to the shaft. At the same time, end spacers and lamination end protectors or spiders may be formed integral with the insulator.
While this provides an excellent solution to the problem with the rotor core securely fixed to the shaft, it is time consuming and any change to the rotor core, such as diameter or length, requires a new mould to be produced.
A second solution involves placing the rotor core on a pre-fabricated plastics material tube. The shaft is then pressed into the tube which expands and locks the core to the shaft. This is effective and allows the size of the rotor core to be changed. However, the process requires significant force to press in the shaft to expand the tube and the longer the tube, the more difficult it is. Also, as the shaft is pressed into the tube, the shaft cannot be keyed to the tube. Under severe conditions of torque and vibration, the rotor core has been known to slip with respect to the shaft. Exact tolerances are required for the rotor core i.d., the shaft o.d. and the tube""s i.d. and o.d. The smaller the rotor and shaft, the more exactly the tolerances are, giving rise to extensive post forming machining to meet the tolerance requirements to avoid slippage.
Thus, there is a need for a double insulated rotor which is easy to assemble and can accommodate variations in the rotor size, i.e., length and diameter.
Accordingly, in one aspect thereof, the present invention provides a double insulated rotor for an electric motor comprising: a shaft; an insulating sleeve moulded to the shaft; a stack of stamped laminations forming a rotor core, the rotor core having a plurality of salient poles and a central aperture receiving the shaft and insulating sleeve; a commutator mounted on the shaft; windings wound around the salient poles and terminated on terminals of the commutator, wherein: the insulating sleeve is injection moulded onto the shaft and has a substantially polygonal outer cross-section where it contacts the rotor core; and the aperture in the rotor core has a corresponding profile adapted to receive the shaft and insulating sleeve as a press fit.
According to a second aspect, the present invention provides a method of making a double insulated rotor for an electric motor, comprising the steps of: injection moulding an insulating sleeve onto a shaft; stacking a plurality of stamped motor laminations to form a rotor core having a plurality of poles and a central aperture; pressing the shaft and sleeve through the central aperture of the rotor core to position the rotor core onto a core portion of the insulating sleeve; mounting a commutator onto the sleeve adjacent the rotor core; winding coils about the poles of the rotor core and connecting the coils to terminals of the commutator, wherein the core section is formed with a substantially polygonal cross-section and the central aperture is formed with a corresponding profile.
Preferred and/or optional features are set out in the dependent claims.